Solids-stabilized oil-in-water emulsion and a method for preparing same

ABSTRACT

A solids-stabilized oil-in-water emulsion and a method for preparing the solids-stabilized oil-in-water emulsion. The oil-in-water emulsion is formed by combining oil, water, solid particles and a pH enhancing agent and mixing until the solid-stabilized oil-in-water emulsion is formed. The low viscosity oil-in-water emulsion can be used to enhance production of oil from subterranean reservoirs. The low viscosity oil-in-water emulsion can also be used to enhance the transportation of oil through a pipeline.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/319,752, filed Dec. 13, 2002, now U.S. Pat. No. 6,988,550, whichclaims the benefit of U.S. Provisional Application No. 60/341,492 filedDec. 17, 2001.

FIELD OF INVENTION

This invention relates to a solids-stabilized oil-in-water emulsion anda method for preparing same. In particular, the invention relates to amethod for reducing the effective viscosity of oil by incorporating theoil into a solids-stabilized oil-in-water emulsion.

BACKGROUND OF INVENTION

Recovery of oil from a reservoir usually results in simultaneousproduction of water with the oil. In many cases the oil and water aresubject to mixing and shearing in subsurface pumps, and this results inthe formation of water-in-oil or oil-external emulsions having aviscosity that is substantially higher than that of the original, “dryoil”. Because of wellbore hydraulics, the production of thisoil-external emulsion, with its higher viscosity, increases liftingcosts (larger pumps and more electrical power requirements) and oftenlimits the production rate from the well, which reduces economicprofitability. Often, demulsifier chemicals are added to thesubterranean formation to either prevent emulsion formation or to breakthe oil-external, high viscosity emulsion. The added demulsifierchemicals are expensive specialty products and need to be customized tothe oil, emulsion and reservoir characteristics in order for the desiredperformance to be achieved. What is needed is a simple, economic methodfor reducing the viscosity of the oil-water mixture.

Moreover, in some cases the original oil is so viscous, such as withsome heavy oils, that even if no water is produced from the reservoirand no oil-external emulsion is formed, the production rate of the oilis nonetheless limited because of its high viscosity. Accordingly, asimple, economic method for reducing the effective viscosity of highlyviscous oil is also needed.

A related problem in the production of oil is a need to obtain anincreased flowrate of the oil through a pipeline, for example, apipeline used to transport oil from the point of production to points ofcollection, transportation, or sale. The viscosity of the oil is alimiting factor in the efficient transportation of oil. As the viscosityof the oil increases, so do the related costs of transportation, such aspumping costs. Existing methods for increasing pipeline capacity are toheat the oil, dilute the oil with less-viscous hydrocarbon diluents,treat the oil with drag reducers, transport the oil in a core annularflow, or convert the oil into an oil-in-water (or water-external)emulsion having a viscosity lower than that of the dry oil. Methods formaking water-external emulsions include adding expensive surfactants oradding surfactants simultaneously with raising the pH of the water-oilmixture by adding base such as sodium hydroxide or ammonium hydroxide.However, for many oils, these treatments do not result in emulsions thatremain sufficiently stable for the long times needed to transport theoil to market. A need exists for an inexpensive method for making awater-external emulsion that remains stable for long periods of time,and can be easily and economically demulsified and separated into theconstituent oil and water.

SUMMARY OF INVENTION

This invention is a solids-stabilized oil-in-water emulsion, and amethod for preparing a solids-stabilized oil-in-water emulsion.

In an embodiment of the invention, the solids-stabilized oil-in-wateremulsion is used to enhance the production of oil from a subterraneanreservoir.

In another embodiment of the invention, the solids-stabilizedoil-in-water emulsion is used to enhance the transportation of oilthrough pipelines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a ternary diagram that illustrates some, but not all, of theparticle shapes that could be characteristic of the solid particles usedto make the solids-stabilized oil-in-water emulsion of this invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a solids-stabilized oil-in-water emulsion and amethod of making a solids-stabilized oil-in-water emulsion. To make asolids-stabilized oil-in-water emulsion pursuant to this invention,solid particles and a pH enhancing agent are added to water and mixedwith oil until the solids-stabilized oil-in-water emulsion is formed.

The solid particles useful for this invention should have certainphysical properties. If the solid particles are to be used in a poroussubterranean formation, as will be explained in more detail, the averageparticle size should be smaller than the average diameter of the porethroats in the porous subterranean formation. At least one sizemeasurement dimension of the actual individual particle size should besufficiently small to provide adequate surface area coverage of thedroplets of oil that are formed within the external water phase.Particle size can be measured by a wide array of particle sizeanalytical techniques, including laser light scattering, mesh screenclassification, Coulter counting method, and settling velocity (whichuses Stokes law to convert a solid sample's settling velocity in a fluidto an average particle size). However, each of these techniques producesan “effective” particle diameter, which is the result that would havebeen produced by corresponding test sample comprised of particles with aspherical shape. Consequently, a particle's effective diameter becomes aless accurate approximation of its true size as the particle's shapedeviates further from a spherical shape. In most instances, however,particles are often irregular and nonuniform in shape.

Without intending to limit the scope of the invention, FIG. 1illustrates this point with a ternary diagram, 114, having threefundamental shape groups. The first group is a plate or pie shape, 102and 104; the second is a bar or cylinder shape, 106 and 108, and thethird is a cube or sphere shape, 110 and 112. Typically, particlescomposing the solids used for making a solids-stabilized emulsiondisclosed herein will have some composite irregular shape that issomewhere between the two or three basic shape groups illustrated internary diagram, 114. Accordingly, the size of particles composing suchsolids are preferably determined using a scanning probe microscopy (SPM)technique. One example of such a technique is atomic force microscopy.Digital Instruments of Santa Barbara, Calif. manufactures an atomicforce microscope (AFM) known as the Nanoscope Multimode™, which has beenused to characterize the average size and shape of some of the solidparticles used in the working examples disclosed below.

Using AFM or some other SPM technique the maximum dimensions of aparticle along its x, y, and z axes can be determined. Therefore, unlessreference to an alternative particle size analysis method is otherwiseindicated, reference to a particle size will mean the smallest of thethree dimensions measured along a particle's x, y, and z axis, asmeasured by a SPM technique. In the case of a perfect sphere, 112, orcube, 110, each dimension is equal while in the case of a particlehaving the shape of a pie, 104, or plate, 102, the thickness of theparticle, as measured along the z axis, is small relative to it length,x, and width y. The “average” particle size for a particular sample canbe determined by obtaining a sufficient number of measurements,preferably 50 or more, of the smallest dimension for the array ofparticles being analyzed. The average size can be calculated usingeither the number of particles among the total measured having aparticular x, y, or z value, whichever is smallest, or the weightcontribution of the particles having a particular x, y, or z value,whichever is smallest, among the total weight for all particlesmeasured.

If spherical in shape, the solid particles should preferably have anaverage size of about ten microns or less in diameter, more preferablyabout two microns or less, even more preferably about one micron or lessand most preferably, 100 nanometers or less. If the solid particles arenon-spherical in shape, they should preferably have an average surfacearea of about 200 square microns or less, more preferably about 20square microns or less, even more preferably about ten square microns orless and most preferably, one square micron or less. The solid particlesmust also remain undissolved in both the oil and water phase of theemulsion under the conditions for which the emulsion is used.

The preferred solid particles are hydrophilic in nature, which includebut are not limited to hydrophilic exfoliated clay, silica, carbonaceoussolids and mixtures thereof. Hydrophilic silica, for exampleAerosil-130™ sold by Degussa Corp. can also be used. Carbonaceous solidslike refinery generated coke can also be used to preparesolids-stabilized oil-in-water emulsions. Refinery coke is generally ahydrophilic carbonaceous solid that can be ground to a powder ofsufficient size for this invention. Montmorillonite clays, for exampleBentonite and Kaolinite clays are also suitable for preparing thesolids-stabilized oil-in-water emulsion. Bentonite clay is suitablebecause it can be easily exfoliated or divided by methods known in theart for exfoliation or division of clays. As mined, bentonite claysnaturally consist of aggregates of particles that can be dispersed inwater and broken up by shearing into units having average particle sizesof 2 microns or less. However, each of these particles is a laminatedunit containing approximately 100 layers of fundamental silicate layersof 1 nm thickness bonded together by inclusions of atoms such as calciumin the layers. By exchanging the atoms such as calcium by sodium orlithium (which are larger and have strong attractions for watermolecules in fresh water), and then exposing the bentonite to freshwater, the bentonite can be broken into individual 1 nm thick layers,called fundamental particles. The chemistry of this delamination processis well known to those skilled in the art of clay chemistry. The resultof this delamination process is a gel consisting of divided bentoniteclay.

Also, the source of the solids used for making a solids-stabilizedoil-in-water emulsion may be indigenous to the formation where suchemulsion is used, hereinafter known as formation solids or formationsolid particles, or may be obtained external to the formation, whethertaken from another formation, mined, or synthesized, hereinafter knownas nonformation solids. In certain instances, in fact, both formationand nonformation solids may be compositionally similar, but simplyderived from different sources.

The pH enhancing agent can be any agent that will raise the pH of thefinal emulsion, preferably to a range of about 7.5 to about 10. It isbelieved that if the final emulsion is not of a sufficient basicity, thesolid particles which are initially hydrophilic (and provide stabilityto the water-external emulsion) gradually become hydrophobic because ofslow absorption of polar hydrocarbons from the oil. A solids-stabilizedoil-in-water emulsion formed without sufficient basicity will eventuallyinvert to a water-in-oil (or oil-external) emulsion, which isundesirable for this invention. The preferred pH enhancing agent forthis invention is a base, or mixture of bases. Preferred basic solutionsinclude sodium hydroxide, potassium hydroxide, ammonium hydroxide,tertiary butyl ammonium hydroxide or mixtures thereof. A typical treatrate of a basic solution is between about 0.005 wt % to about 5.0 wt %based on the weight of the water.

The water used to form the emulsion preferably contain salts of Group Iand Group II elements. Chlorides, sulfates, carbonates are some of thecommonly occurring salts in water. Presence of these and other saltshave a beneficial effect on emulsion formation and stability.

To make the solids-stabilized oil-in-water emulsion in accordance withthis invention, oil can be added to water that contains the solidparticles and the basic solution. This method is preferred where thesolids-stabilized emulsion is prepared above ground, for example, in asurface facility. Alternatively, water containing the solid particlesand the basic solution can be added to the oil, which is preferred forsubterranean preparation of a solids-stabilized oil-in-water emulsion.The severity and duration of mixing required to form thesolids-stabilized oil-in-water emulsion can be determined by one ofordinary skill in the art based upon such factors as the oil viscosityand oil composition. Mixing in a surface facility can be accomplishedusing static mixers (e.g., paddle mixers), blade mixers, inline mixers(e.g., plurality of fins in a pipe), or propagation of the oil-watermixture through an orifice. In a subsurface formation of emulsions, themixing occurs in the subsurface pumps. The percentage of oil in waterfor the solids-stabilized oil-in-water emulsion can vary between 10% to90%. The preferred treat rate of solid particles in the emulsion isbetween about 0.01 wt % to about 5.0 wt % based upon the weight of theoil.

It is preferred to have oil droplets in the size range of 1 micron to200 microns in diameter suspended in the external water phase. Ingeneral, the dispersed oil droplets size distribution can be controlledby increasing the rate and/or duration of mixing and by increasing theconcentration of solid particles and/or the pH enhancing agent.Conventional optical microscopy can be used to observe the oil dropletsdispersed in water. Alternate methods to determine the oil droplet sizedistribution are known in the art, and can be used, such as the CoulterCounter method.

The following laboratory experiment and prophetic examples are intendedto illustrate examples of making and using the solids-stabilizedoil-in-water emulsions of this invention. However, to the extent thatthe following descriptions are specific to a particular embodiment or aparticular use of the invention, this is intended to be illustrativeonly and is not to be construed as limiting the scope of the invention.On the contrary, it is intended to cover all alternatives,modifications, and equivalents that are included within the spirit andscope of the invention, as defined by the appended claims.

Laboratory Experiment—Reduction of Effective Viscosity of Oil

The viscosity of a sample of dry crude oil was measured to be 320 cp at140° F. using a Brookfield viscometer. An oil-external emulsioncontaining 30 volume % (vol. %) water was made with this oil sample,which increased the effective viscosity of the oil (i.e. the viscosityof the water-in-oil emulsion) to 940 cp (140° F. at 380 sec⁻¹ shearrate). A water-external emulsion was made with this crude oil sample,also containing 30 vol. % water, by mixing bentonite solid particles (ata treat rate of between 0.01 wt % to about 5 wt % based on the weight ofthe oil), and 100 ppm of ammonium hydroxide (to bring the pH of thefinal emulsion to a range of about 7.5 to 10) to the water and oil, andshearing until the water-external emulsion was formed. The resultingwater-external emulsion exhibited a viscosity of 91 cp (140° F. at 380sec⁻¹ shear rate), which is a viscosity reduction of 229 cp from the 320cp viscosity of the dry oil.

Production of Oils Using a Solids-Stabilized Oil-in-Water Emulsion.

An aqueous solution comprising a volume of water containing a pHenhancing agent (for instance a basic solution like ammonium hydroxide),and hydrophilic solid particles (such as bentonite that has beenexfoliated into individual, fundamental layers), is pumped into asubterranean oil formation from the surface. The aqueous solution isprovided within the subterranean formation at a location so that itcontacts the produced oil and connate water (if any) at a depth belowwhere the fluids enter the submersible pump. The aqueous solution,connate water and oil are mixed within the subterranean formation bypropagation of the mixture through pores in the formation and/or mixingin the submersible pump thereby forming the solids-stabilizedoil-in-water emulsion. The solids-stabilized oil-in-water emulsion has alower viscosity than the original dry oil and a lower viscosity than thewater-in-oil emulsions that can be formed in the formation by mixing ofthe dry oil and connate water. The low viscosity solids-stabilizedoil-in-water emulsion can be produced from the subterranean formationusing production methods commonly known in the industry.

The amount of solid particles and basic solution added to the injectedwater to form the aqueous solution can be determined by one skilled inthe art pursuant to the particular characteristics of the subterraneanreservoir and oil being produced. The produced oil-in-water emulsionpreferably has a final concentration of solid particles in the range ofabout 0.01 wt % to about 5 wt % based on the weight of the oil. Theamount of basic solution added to the injected water should besufficient to raise the pH of the resulting oil-in-water emulsion toabove 7.0, and preferably to a range of 7.5–10. The amount of baseneeded depends on the acid content (Total Acid Number, TAN) of the oil.For example, for oils whose TAN ranges from 4–7, the amount of ammoniathat needs to be added is about 100–200 ppm in the final emulsion.Generally, it is preferred to add an amount of base sufficient toneutralize at least 25% of the acids. The oil-in-water emulsion producedby addition of solid particles and the basic solution will usuallyremain stable for longer periods of time and have a lower viscosity thanan emulsion made by addition of a basic solution alone. Moreover, theresulting water-external emulsion has a substantially lower viscositythan the dry oil alone, and can be lifted at lower cost and at higherrate than either the dry oil alone or the oil-external emulsions usuallyformed in the wellbore.

Once produced, the solids-stabilized water-external emulsion can beallowed to gravity settle in surface facilities to concentrate the oilcontent to approximately 70–80 vol. % oil. The excess water can beremoved after separation. This water-external emulsion containing 70–80vol. % oil is well suited for transport in pipelines as described below.To break the water-external emulsion, a sufficient pH reducing agent(such as an acidic solution) can be added to reduce the pH of theemulsion to a value below about 7.0. Water can then be removed byconventional dehydration methods such as using electrostatic coalescencedevices or hydrocylones.

Transportation of Oil Using a Solids-Stabilized Oil-in-Water Emulsion.

Water-external emulsions can be transported in pipelines to achievehigher net flowrates of oil than in the transport of dry oil alone. Thepercentage of oil in water for this embodiment can vary between 10% to90%, and is preferably in the range of 70 to 80%. In a particularexample, oil is combined with an aqueous solution comprising water, a pHenhancing agent (such as a basic solution), and solid particles andmixed until the solids-stabilized oil-in-water emulsion is formed. Theamount of basic solution is preferably in a range between about 0.005 wt% to about 5 wt % based upon the weight of the water. The pH of theresulting oil-in-water emulsion should be above 7.0 and preferably be inthe 7.5–10 range. The solid particles can be added at a treat range ofabout 0.01 wt % to about 5 wt % based upon the weight of the oil.

The oil useful for this embodiment of invention can be any oil includingbut not limited to crude oil, crude oil distillates, crude oil residue,synthetic oil, and mixtures thereof.

In propagating the emulsion through a pipe it is preferred to firstcontact the inner walls of the pipe with a wettability altering agent tomake the inner walls of the pipe water-wet to aid the propagation of theoil-in-water emulsion. The wettability altering agent can be water oranother known drag reducer that can be selected by one of ordinary skillin the art. After contacting the inner walls of the pipe with thewettability altering agent, the oil-in-water emulsion can be pumpedthrough the pipe.

High-oil content, solids-stabilized oil-in-water emulsions are thereforegood candidates for transportation in pipelines using flow regimes ofeither self-lubricating core annular flow or as uniform, lower-viscositywater-external emulsions. In core annular flow, forming a low-viscosityannulus near the pipe wall further reduces pressure drop. Because theviscosity of a solids-stabilized oil-in-water emulsions is not greatlyaffected by temperature (because the viscosity of water, the continuousphase, is not greatly affected by temperature), such oil-in-wateremulsions do not have to be heated to high temperature to maintain anacceptably low viscosity for economical transport. Another benefit ofthe oil-in-water emulsion formed in the above manner is that the oilphase does not tend to wet steel. Thus, these emulsions will have fewertendencies to wet or “foul” the pipeline walls than oil-externalemulsions or dry oil.

The present invention has been described in connection with itspreferred embodiments. However, to the extent that the foregoingdescription was specific to a particular embodiment or a particular useof the invention, this was intended to be illustrative only and is notto be construed as limiting the scope of the invention. On the contrary,it was intended to cover all alternatives, modifications, andequivalents that are included within the spirit and scope of theinvention, as defined by the appended claims.

1. A method for transporting oil through a pipe comprising: preparing asolids-stabilized oil-in-water emulsion by: combining water, oil, micronto sub-micron sized hydrophilic solid particles and a pH enhancingagent; and mixing said combination until said solid-stabilizedoil-in-water emulsion is formed; and transporting said solids-stabilizedoil-in-water emulsion through said pipe.
 2. The method of claim 1,wherein said hydrophilic solid particles are combined at a treat rangeof about 0.01 wt % to about 5 wt % based upon the weight of said oil. 3.The method of claim 1, wherein said hydrophilic solid particles comprisespherically shaped solid particles with an average particle size of lessthan about 10 microns in diameter.
 4. The method of claim 1, whereinsaid hydrophilic solid particles comprise non-spherically shaped solidparticles with an average surface area of less than about 200 squaremicrons.
 5. The method of claim 1, wherein said pH enhancing agentprovides said emulsion with a pH level in a range of about 7.5 to about10.0.
 6. The method of claim 1, wherein said pH enhancing agentcomprises a basic solution.
 7. The method of claim 6, wherein said basicsolution is selected from the group consisting of sodium hydroxide,potassium hydroxide, ammonium hydroxide, tertiary butyl ammoniumhydroxide and mixtures thereof.
 8. The method of claim 6, wherein saidbasic solution is provided at a concentration of about 0.005 wt % toabout 5 wt % based on the weight of the water.
 9. The method of claim 1,further comprising contacting the inner walls of said pipe withwettability altering agent prior to said step of transporting saidsolids-stabilized oil-in-water emulsion.
 10. The method of claim 9,wherein said wettability altering agent comprises water.